High Brightness LED Controller
using the MC9RS08KA2
Designer Reference Manual
RS08
Microcontrollers
DRM080
Rev. 0
5/2006
freescale.com
High Brightness LED Controller using the MC9RS08KA2
Designer Reference Manual
by: Dennis Lui, Vincent Ko
Freescale Semiconductor, Inc.
Hong Kong
To provide the most up-to-date information, the revision of our documents on the World Wide Web will be
the most current. Your printed copy may be an earlier revision. To verify that you have the latest
information available, refer to http://www.freescale.com
The following revision history table summarizes changes contained in this document. For your
convenience, the page number designators have been linked to the appropriate location.
Revision History
Date
Revision
Level
05/2006
0
Description
Initial release
Page
Number(s)
N/A
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
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Revision History
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
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Freescale Semiconductor
Table of Contents
Chapter 1
Introduction
1.1
1.2
1.3
1.4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Freescale’s New Generation Ultra Low Cost MCU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
7
7
8
Chapter 2
Control Theory
2.1
2.2
Switching Regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Buck Converter Basics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Chapter 3
Hardware Implementation
3.1
3.2
3.3
3.3.1
3.3.2
3.3.3
3.3.4
3.3.5
3.4
3.4.1
3.4.2
3.4.3
3.4.4
3.4.5
3.4.6
3.4.7
3.4.8
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Functional Pin Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hardware Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PWM Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Feedback Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reference Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
User Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Background Debug Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Design Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High Brightness LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Current Sense Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RC Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inductor Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
P-Channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Schottky Diode Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PCB Layout Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
13
13
14
15
16
17
17
17
17
17
17
18
18
18
18
18
Chapter 4
Software Implementation
4.1
4.2
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Software Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Main Software Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
KBI Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Feedback Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PWM Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
19
19
20
20
21
22
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
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Table of Contents
Chapter 5
Testing
5.1
5.2
5.2.1
Measurement Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Customization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Chapter 6
Demo Setup
6.1
6.2
6.3
6.4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Default Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Programming and Running the HB-LED Demo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
25
25
26
Appendix A.
Schematic
Appendix B.
Program Listing
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
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Freescale Semiconductor
Chapter 1
Introduction
1.1 Introduction
This document describes a reference design of a high brightness LED control solution using the
MC9RS08KA2 microcontroller.
Recently, light emitting diodes (LEDs) have become very popular in general lighting area as a
replacement technology for halogen low-voltage lighting. Customers are quickly recognizing the
advantages of using LED lighting, which include long operating life, no fragile glass, no mercury, and low
voltage DC operated.
In general, LEDs have a nonlinear I-V behavior and thus current limitation is required to prevent the power
dissipation to exceed a maximum limit. Thus, the ideal source for LED driving is a constant current source.
The concept of this application is a MCU based LED driver with closed-loop current control. A compact
LED light source with dimming control is implemented to demonstrate the advantages of using MCU to
drive a high brightness LED with different average current settings.
All hardware schematic diagrams and firmware source codes are available as reference materials.
1.2 Features
•
•
•
•
•
•
High brightness LED driver with 350mA current driving capability
Control a buck converter to regulate supply voltage to match with different LED forward voltages
Up to 80% efficiency
Internally generated PWM switching frequency
Feedback control on LED forward current through a current sense resistor
Dimming control by a single button
1.3 System Overview
A block diagram of the system is shown in Figure 1-1.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
7
Introduction
RS08KA2
User
Interface
KBI
PWM
DC/DC
Converter
PWM
USB
To PC
Feedback
OS BDM
MCU
Control
HB-LED
ACMP
GPIO Port
Reference
Setting
Figure 1-1. System Block Diagram
1.4 Freescale’s New Generation Ultra Low Cost MCU
The MC9RS08KA2 (KA2) microcontroller unit (MCU) is an extremely low cost, small pin count device for
home appliances, toys, small geometry, and LED control applications. This device is composed of
standard on-chip modules including a very small and highly efficient RS08 CPU core, 62 bytes RAM, 2K
bytes FLASH, an 8-bit modulo timer, keyboard interrupt, and analog comparator. The device is available
in small 6- and 8-pin packages.
MC9RS08KA2 Features:
• Simplified S08 instruction set with added high-performance instructions
• 2048 bytes on-chip FLASH EEPROM
• 62 bytes on-chip RAM
• Internal clock source
• Up to 10-MHz internal bus operation
• Background debug system
• Power-saving modes
• Low-voltage detection
• 8-bit modulo timer
• Analog comparator
• Keyboard interrupt ports
Timer system features include:
• 8-bit up-counter
– Free-running or 8-bit modulo limit
– Software controllable interrupt on overflow
– Counter reset bit (TRST)
– Counter stop bit (TSTP)
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
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Freescale Semiconductor
Freescale’s New Generation Ultra Low Cost MCU
•
•
Four software selectable clock sources for input to prescaler:
– System bus clock — rising edge
– Fixed frequency clock (XCLK) — rising edge
– External clock source on the TCLK pin — rising edge
– External clock source on the TCLK pin — falling edge
Nine selectable clock prescale values:
– Clock source divide by 1, 2, 4, 8, 16, 32, 64, 128, or 256
The analog comparator has the following features:
• Full rail-to-rail supply operation
• Less than 40 mV of input offset
• Less than 15 mV of hysteresis
• Selectable interrupt on rising edge, falling edge, or either rising or falling edges of comparator
output
• Option to compare to fixed internal bandgap reference voltage
• Option to allow comparator output to be visible on a pin, ACMPO
• Remains operational in stop mode
The KBI features include:
• Each keyboard interrupt pin has individual pin enable bit
• Each keyboard interrupt pin is programmable as falling edge (or rising edge) only, or both falling
edge and low level (or both rising edge and high level) interrupt sensitivity
• One software-enabled keyboard interrupt
• Exit from low-power modes
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
9
Introduction
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
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Freescale Semiconductor
Switching Regulator
Chapter 2
Control Theory
2.1 Switching Regulator
A switching regulator regulates a current flow by chopping up the input voltage and controlling the average
current by means of the duty cycle. When a higher load current is required by the load, the percentage of
on-time is increased to accommodate the change. There are two basic types of switching regulators:
forward-mode regulators and flyback-mode regulators. The name of each type is derived from the way
the magnetic elements are used within the regulator.
Forward-mode switching regulators have four functional components: a power switch, a rectifier, a series
inductor, and a capacitor (see Figure 2-1). The power switch may be a power transistor or a metal oxide
semiconductor field-effect transistor (MOSFET) placed directly between the input voltage and the LC filter
section. The shunt diode, series inductor, and shunt capacitor form an energy storage tank whose
purpose is to store enough energy to maintain the load voltage and current over the entire off-time of the
power switch. The power switch serves only to fill up the energy lost to the load during its off-time.
Flyback-mode switching regulators have the same four basic elements as the forward-mode regulators
except that they have been rearranged in another configuration. In this design, forward-mode switching
is used.
SW1
Vin
L
D
Co
RL
Figure 2-1. Forward Mode Switching Regulator
2.2 Buck Converter Basics
A “buck” or step-down converter is the most elementary forward-mode converter. Its basic schematic can
be seen in Figure 2-1.
The operation of this regulator topology has two distinct time periods. The first one occurs when the series
switch SW1 is on, the input voltage Vin is connected to the input of the inductor L. The output of the
inductor is the output voltage, and the rectifier (or catch diode) is reverse biased. During this period, since
there is a constant voltage source connected across the inductor, the inductor current begins to linearly
ramp upwards, as described by the following equation:
I L ( on ) = [ ( V in – V out ) × t on ] ⁄ L
(EQ 2-1)
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
11
Control Theory
During this “on” period, energy is stored within the core material in the form of magnetic flux. If the inductor
is properly designed, there is sufficient energy stored to carry the requirements of the load during the “off”
period.
The next period is the “off” period of the power switch. When the power switch turns off, the voltage across
the inductor reverses its polarity and is clamped at one diode voltage drop below ground by the catch
diode. The current now flows through the catch diode thus maintaining the load current loop. This
removes the stored energy from the inductor. The inductor current during this time is:
I L ( off ) = [ ( V out – V D ) × t off ] ⁄ L
(EQ 2-2)
This period ends when the power switch is once again turned on. Regulation of the converter is
accomplished by varying the duty cycle of the power switch according to the loading conditions. To
achieve this, the power switch requires electronic control for proper operation. It is possible to describe
the duty cycle as follows:
d = t on ⁄ T
(EQ 2-3)
where T is the switching period.
For the buck converter with ideal components, the duty cycle can also be described as:
d = V out ⁄ V in
(EQ 2-4)
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
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Freescale Semiconductor
Introduction
Chapter 3
Hardware Implementation
3.1 Introduction
The system consists of the 8-pin packaged KA2 and external components. There are four major external
components: P-channel MOSFET, schottky diode, inductor, and capacitor which are used for switching
regulation.The KA2 is used as a PWM controller to control the HB-LED brightness level. A simple DC to
DC step-down converter (buck topology) is implemented to convert the 5V input supply to a current
regulated output for LED driving. The integrated open source BDM (background debug mode) design
allows users to program the MCU FLASH and debug applications via a USB connection.
OS-BDM
KA2 Control
HB-LED
Figure 3-1. High Brightness LED Control Board
3.2 Functional Pin Assignment
•
•
•
•
•
•
PA0 – Reference Voltage Input
PA1 – Feedback Input
PA2 – Push Button Input
PA3 – Dimming Control DIM0
PA4 – PWM Output
PA5 – Dimming Control DIM1
3.3 Hardware Functions
Figure 3-2 shows a block diagram of the HB-LED reference design. The hardware functions can be
divided into the following parts:
• PWM Control
• Feedback Loop
• Reference Voltage
• User Interface
• Background Debug Interface
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
13
Hardware Implementation
VDD = 5V
RS08KA2
KBI
PWM
(MTIM)
KBI
R4
Q1
PMOS
L
PWM
Vout
SW
D
ACMP
Co
Rf
FB
MCU
Control
Cf
Vref
HB-LED
VDD
Rs
R1
VDD
R2
GND
R3
DIM0
DIM1
Figure 3-2. Functional Block Diagram
3.3.1 PWM Control
A fixed frequency step-down (buck) DC to DC converter is implemented with the KA2 for usage in high
brightness LED driving applications. The KA2 consists of all active functions required to directly
implement a step-down converter with a minimum number of external components. The KA2 enables
control of PWM on power switch device to regulate the LED current. The PMOS device ON/OFF switching
is controlled by MCU timer module and the frequency is set to around 30kHz. Base the on analog
comparator output result, the MCU adjusts the PWM duty cycle in closed-loop to keep the LED driving
current as a constant. Since a P-channel MOSFET device is used as the power switch, the PWM control
signal should be generated in active low polarity, that means the power switch will turn on if the PWM
signal is in LOW level.
A switching cycle is initiated by the falling edge of the PWM signal. At the moment the power switch Q1
is turned on, the inductor is connected to VDD and the inductor current begins to ramp up linearly. When
the PWM signal is in HIGH state, the power switch Q1 turns off and the current flows through the diode D
to maintain the current loop. The capacitor CO filters the regulated output and provides the converter loop
stability. The converter is designed to operate in continuous conduction mode, which means the inductor
current will not decrease to zero before the new cycle start. The average inductor current IL is almost
equal to the average loading current IO.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
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Freescale Semiconductor
Hardware Functions
Inductor Current
IL
IO
Time
T = 33us
PWM
Output
PMOS
ON
OFF
ON
OFF
ON
OFF
Time
Figure 3-3. PWM Control Waveforms
The external pull-up resistor R4 ensures the PMOS device Q1 is in OFF state during power up period,
and prevents a high current surge through the LED before MCU controls the system.
3.3.2 Feedback Loop
The KA2 features an analog comparator which can be used to determine the LED current. The inverting
input pin and non-inverting input pin are used for current sense feedback input and reference voltage input
respectively.
Current limiting is implemented by monitoring the LED forward current, which is one of the key parameters
affecting the performance of LED, particularly on the operating life time, so it is important to control and
make sure the average current and peak current are also within the limits specified in the LED
specification.
The LED forward current is converted to a voltage by using an external resistor connected in series with
the LED. The voltage across the current sense resistor RS is directly proportional to the current through
LED. This feedback voltage is compared against a reference by the analog comparator. When the
feedback voltage is higher than the level at reference input, the PMOS switch ON time reduces to limit
the current accumulated at the inductor L. In contrast, the PMOS switch ON time increases again to pump
more current into the inductor when the feedback voltage is lower than the reference level. The upper and
lower threshold levels are defined by the hysteresis range of the comparator.
Feedback Voltage
VRs
Hysteresis
ACMP Ref
Time
PWM
Output
decrease
ON time
increase
ON time
Time
Figure 3-4. Feedback Control Waveforms
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
15
Hardware Implementation
In steady-state condition, the LED forward current ILED is equal to the reference voltage VREF divided by
the current sense resistor RS.
The LED forward current is determined by the following equation:
V REF
I LED = ------------RS
(EQ 3-1)
A simple low-pass filter is added between the current sense resistor RS and the feedback path to eliminate
any high frequency noises coupling into the feedback pin, However, the response time of the filter should
be fast enough to track the deviation of the voltage across the current sense resistor RS.
In this reference design, the PWM switching frequency is around 30kHz and the filter cut-off frequency is
set to 5kHz.
The following equation shows the relationship between the filter cut-off frequency and the RC values:
1 f = -------------2πRC
(EQ 3-2)
3.3.3 Reference Voltage
The reference voltage in the comparator non-inverting input is determined by a resistor network which
consists of three resistors and the connection paths are controlled by the MCU. The pin DIM0 is always
configured as output pin and set to output LOW. The resistor R2 connects to ground and generates a
reference voltage divided from VDD. This is the default reference voltage and corresponds to 100%
brightness level. When the pin DIM1 is set to output LOW, R3 is also connected to ground and in parallel
with R2, the reference voltage will be further reduced to a lower level. The DIM1 pin should be configured
as input state if it is not used in dimming selection.
The equations for determining the reference voltage with the divider network are:
For High brightness:
R2
V REF = V DD ⎛⎝ -------------------⎞⎠
R1 + R2
(EQ 3-3)
Rp
V REF = V DD ⎛ -------------------⎞
⎝ R 1 + R p⎠
(EQ 3-4)
For Low brightness:
where Rp is equal to R2 parallel with R3.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
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Freescale Semiconductor
Design Procedures
3.3.4 User Interface
A single push button, SW, is used as user interface to toggle the LED brightness level between 100% and
40% (default brightness level is 100%). The dimming control is accomplished by changing the LED
forward current which is set by the reference voltage applied at the comparator’s non-inverting input.
Table 3-1. Dimming Control
Control Pins
100% Brightness
40% Brightness
DIM0
Output LOW
Output LOW
DIM1
Hi-Z
Output LOW
3.3.5 Background Debug Interface
The on-board open source BDM provides a cost effective debug solution for Freescale RS08 MCUs. The
module utilizes the modified version of popular open-source BDM solution from the Internet for in-circuit
emulation and device programming. The board comprises a 12V DC-DC converter for RS08 FLASH
programming support.
NOTE
In circuit debug can not be used for this application program since the
BKGD pin is used for dimming control purpose. Use JP3 to switch between
BDM and user mode.
3.4 Design Procedures
This section presents guidelines for selecting external components.
3.4.1 High Brightness LED
The system is designed to drive a high brightness LED with typical 350mA forward current at 3.5V forward
voltage, i.e. the power dissipation on the LED is around 1.2W.
3.4.2 Current Sense Resistor
The value of the current sense resistor RS is determined by two factors: power dissipation on RS and the
threshold level for comparator input. Smaller RS reduces power dissipation but the detection of feedback
signal in comparator is more difficult.
Base on equation (EQ 3-1), setting RS to 1Ω, the feedback voltage is equal to 350mV.
Power dissipation on RS is around 120mW, I2R = (350mA)2 × 1Ω, which is reasonable compared to LED
power.
3.4.3 RC Network
Base on equations (EQ 3-3) and (EQ 3-4), set VDD = 5V, R1 = 10kΩ, R2 = 750Ω, R3 = 500Ω.
The reference voltage for high and low brightness settings are equal to 350mV and 146mV respectively.
Low brightness setting is corresponds to around 40% brightness level.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
17
Hardware Implementation
3.4.4 Inductor Value
We can determine the inductor value by using the following equations:
di
V L = L × ----dt
where:
(EQ 3-5)
D×T
L = ( V S – V O ) × -------------I ripple
(EQ 3-6)
3.85
1---------- × --------5
30K
L = ( 5 – 3.85 ) × --------------------------- = 337µH
0.35 × 0.25
(EQ 3-7)
VL is inductor voltage, VS is source voltage, VO is output voltage
D is conduction duty cycle, T is conduction period
Iripple is inductor peak-to-peak ripple current, set to 25% of output current
The inductor current rating must be higher than the maximum peak current flowing through the inductor.
3.4.5 Output Capacitor
Low ESR (equivalent series resistance) aluminium or solid tantalum capacitor is recommended for low
output ripple voltage. The ESR of the output capacitor and the inductor ripple current are two major factors
contributing to the output ripple voltage.
Output ripple voltage = Inductor ripple current × ESR
(EQ 3-8)
3.4.6 P-Channel MOSFET
For superior switching performance, a P-channel MOSFET device with low on-state resistance and low
gate charge should be selected. The current rating must be at least 1.2 times greater than the maximum
load current and the drain-to-source breakdown voltage should be at least 1.25 times the maximum input
voltage. The P-channel MOSFET NTR4502 from On Semiconductor is selected in this design.
3.4.7 Schottky Diode Rating
A diode with a high forward voltage drop or long turn-on delay time should not be used, so a fast switching
and low drop schottky diode is selected. The current and reverse voltage rating requirements should be
similar to the P-channel MOSFET device. The schottky diode MBR130 from On Semiconductor is used
in this design.
3.4.8 PCB Layout Guidelines
The PCB layout is very important for switching converter design and is critical to reduce noise and ensure
specified performance. Care should be taken in PCB layout to avoid rapidly switching currents to generate
voltage transients and affect the desired operation.
• Minimize inductance and ground loops, the length of the leads indicated by heavy lines should be
kept as short as possible. (e.g. PMOS / coil / schottky diode.)
• Minimize the trace from the current sense resistor to the feedback input and keep it away from
noise sources to avoid noise pick up.
• Locate the sensitive voltage reference divider network close to MCU.
• Single ground point or ground plane design should be used.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
18
Freescale Semiconductor
Introduction
Chapter 4
Software Implementation
4.1 Introduction
The software is designed with a main loop to monitor the feedback voltage from the LED and generate a
PWM controlled waveform for DC to DC switching operation. The PWM timing is constructed by the
modulo timer overflow periods of PWM ON time & OFF time and the software overhead using for user
interface detection & feedback loop control (see Figure 4-1). The timing used for each branch routine
paths should be keep as constant such that the period of the generated PWM is stable and independent
of the executed routine path.
Period = (S/W overhead + PWM ON) + PWM OFF
PWM
Output
PMOS
OFF
ON
ON
OFF
ON
OFF
Time
1
1.
2.
3.
4.
2
3
4
Set PWM ON; detect KBI, Check ACMP, and adjust ONTime & OFFTime (software overhead)
MTIMMOD = ONTime
MTIM overflow; set PWM OFF; MTIMMOD = OFFTime
MTIM overflow again
Figure 4-1. PWM Timing
4.2 Software Design
This section describes the design of the software blocks. The software description comprises these
topics:
• Main Software Flow
• Initialization
• KBI Detection
• Feedback Comparison
• PWM Generation
4.2.1 Main Software Flow
Figure 4-2 shows the flow of the main program, it also indicates the functions that KA2 are to perform.
The initialization subroutine initializes the modules of real-time interrupt, modulo timer, analog
comparator, and keyboard interrupt.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
19
Software Implementation
Reset
Initialization
Check KBI
Modify Dimming
Check ACMP
Update PWM
PWM Generator
Figure 4-2. Main Software Flow
4.2.2 Initialization
The Initialization routine initializes the MCU with following configurations:
• Initializes ICS and sets bus clock to 10MHz
• Initializes page register, disables COP and BKGD
• Enables LVI, and sets RTI to 128ms
• Initializes RAM variables
• Initializes GPIO
• Initializes KBI
• Initializes the ACMP
• Initializes modulo timer
– Timer prescalar = 1, timer clock = 10MHz
– Maximum of period = 25.6µs
– Timer resolution = 100ns
4.2.3 KBI Detection
The state of the user interface is scanned every 128ms to see if any button press event has occurred.
When a key press event is detected, it sets the corresponding “Pressed” flag. The output state of DIM1
pin will toggle to setup a new reference voltage for ACMP in the next 128ms period. This is to set up a
key debounce period of 128ms.
The real-time interrupt function is used to generate a periodic interrupt for KBI scanning. The RTI is driven
from the 1kHz internal clock reference and the wake-up period is set to 128ms.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
20
Freescale Semiconductor
Software Design
N
RTIF?
Y
N
Pressed?
Y
N
KBIF?
Dim LED
Y
Pressed = 0
Pressed = 1
Figure 4-3. KBI Detection Routine
4.2.4 Feedback Comparison
The function of the feedback comparison routine is to control the PWM duty cycle periodically. When it
detects a crossover point between the feedback voltage and the reference voltage, the ONTime and
OFFTime variables will be updated according to the result of the analog comparator output that is used
as a feedback signal to indicate whether the LED current is higher or lower than the expected level.
These two variables should be adjusted in opposite polarity such the overall period is kept constant (i.e.
OFFTime = 255 – ONTime). The PWM duty cycle is determined by the value of ONTime. The variables
ONTime and OFFTime should be set in the range of 0 to 255.
N
FB < Vref?
Y
N
ONTime<255?
Y
ONTime + 1
N
ONTime>0?
Y
ONTime - 1
Figure 4-4. Feedback Comparison Routine
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
21
Software Implementation
4.2.5 PWM Generation
The 8-bit modulo timer is configured to generate the PWM signal. The timer is running in modulo mode
such that the overflow flag will be set when the count value matches the modulo value in the MTIM module
register, MTIMMOD. PWM output is implemented by programming two timer overflow variables, ONTime
and OFFTime alternately into the MTIM module register.
The overall PWM period is equal to the sum of PWM ON, PWM OFF and the time used for software
looping in KBI detection and feedback comparison routines.
Set PWM ON
Software overhead
ONTime > 0?
N
Y
MTIMMOD
= ONTime
TOF?
Set PWM OFF
N
MTIMMOD
= OFFTime
Y
OFFTime = 0?
N
TOF?
Y
N
Y
Figure 4-5. PWM Generation Routine
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
22
Freescale Semiconductor
Measurement Data
Chapter 5
Testing
The reference design was tested with Lumileds high brightness LED in typical conditions. The LED
forward current was regulated with expected performance with 5V supply input. Dimming control was
achieved by changing the LED current with a single button user interface.
5.1 Measurement Data
Figure 5-1 shows the voltage waveform measured across the current sense resistor RS in 100%
brightness setting. The LED forward current was determined by this voltage divided by one (RS = 1Ω),
which was equal to around 385mA and the peak-to-peak ripple current was 26mA. The measurements
show that the LED forward current was regulated and tracking to the reference input accurately. The
difference between the calculated target (350mV) and measured voltage (385mV) showing in Figure 5-2
is due to the tolerance of resistors used.
Figure 5-1. Waveform at RS
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
23
Testing
Figure 5-2. Waveform at Reference Input
The efficiency of the whole converter was measured as:
1. Supply input = 5V, supply current = 302mA, DC to DC output voltage = 3.5V
2. Voltage across RS = 385mV, LED forward voltage = 3.115V
PowerEfficiency = Output
-------------------------------InputPower
(EQ 5-1)
× 0.385- = 79.4%
Efficiency = 3.115
-------------------------------5 × 0.302
(EQ 5-2)
3. Overall efficiency = 79.4%
5.2 Customization
5.2.1 Hardware
The on-board open source BDM is used for development purposes only. It can be removed to make the
design more compact and fit into a smaller housing.
The inductor value can be adjusted to match with new LED forward current requirement.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
24
Freescale Semiconductor
Introduction
Chapter 6
Demo Setup
6.1 Introduction
This section shows how to setup the MC9RS08KA2 HB-LED demo board and run the included demo
program. Users can use the on-board open source BDM module to program the MCU FLASH and debug
new applications via the USB connection.
6.2 Default Settings
Figure 6-1 shows the default settings for the demo board. The black blocks indicate the default position
of the jumpers. Please check these settings before continuing.
JP1
EXT
USB
BDM USER
JP3
Figure 6-1. Demo Board Default Settings
6.3 Programming and Running the HB-LED Demo
Further application development and debug for the MC9RS08KA2 is supported through the on-board
OS-BDM interface. A USB type B connector (S1) provides the connection between the demo board and
your host PC. This tool is tested to run on Windows XP with Freescale CodeWarrior version 5.1. Use the
procedure below to operate the HB-LED demo board.
The board must be powered by external 5V supply when HB-LED is turned on. Do not use the USB 5V
for powering the HB-LED.
Program:
1. Move jumper JP3 to the 1-2 position “BDM” to power up device in background debug mode.
2. Programme the device with the corresponding software via the USB connection. Refer to
documentation in the CD-ROM that came with the HB-LED demo board.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
25
Demo Setup
NOTE
In-circuit debugging cannot be used for this application program since the
BKGD pin is used for dimming control purpose. Use JP3 to switch between
BDM and user mode.
Run:
1. Move jumper JP3 to the 2-3 position “USER” to isolate the “BDC_BKGD” pin.
2. Power up the device for software execution in user mode.
NOTE
All RS08 devices require a power-on-reset to switch between user mode
and background debug mode.
6.4 Troubleshooting
The high brightness LED does not turn on:
• Make sure jumper JP3 is set to “USER” (2-3) position.
• Make sure supply input selection (JP1) is selected for external power supply.
Unable to program the MCU using the on-board OS-BDM:
• Make sure jumper JP3 is set to “BDM” (1-2) position.
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
26
Freescale Semiconductor
A
B
C
D
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
C16
1nF
GND
2
1
BDC_RESET_VPP
1
27
5
1
3
5
75K
DON’T
POPULATE
J2
4
1
47
C19
1uF
1
Q3
BSS84
T1
3
2
17K(1%)
10K(1%)
2
1
2
1
DON’T
POPULATE
R6
4
R8
R6
2
1
R4
R22
22K
GND
2
1
2
1
R9
600
RV2
20K
MCU_VDD
R17
BDM_DRV
MCU_VDD
2
8
7
6
5
2
1
VCC_USB
1
2
1
4K7
R13
VPP
BDM_IN_OUT
BDM_IN_OUT
PORT_7.5V
PORT_12V
BDM_DRV
2
1
VSS
OSC1
OSC2
VREG
VDD
PTD0
PTD1
PTD2
PTD3
PTD4
PTE1
PTE3/D+
PTE4/DPTC0
U1
R15
1K
3
GND
2
J1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
2
4.7uF
C7
3
2
1
VPP_CTL
BDM_IN_OUT
D+
DBDM_DRV
LED_GREEN
LED_RED
0.1uF
C6
2
1
1K5
1
1
1
2
3
1
JP1
2
1
2
2
1
10
1
SW1
2
3
1
2
470
470
R12
2
1
VREG
0.1uF
C8
2
1
USB CLASS B
SOCKET
VCC
-D
+D
GND
2
2
VCC_USB
MCU_VDD
R11
S1
4.7uF
C17
1
2
3
4
1
1
470
R19
1
1
DATE: 14/04/06
SHEET 01 of 02
C12 CLOSE TO U2 VCCB
C17 & C8 CLOSE TO VREG PIN
2
1
2
GND
RED
D4
2
GREEN
D3
0.1uF
C12
2
VPP
1
USB
1
REV: 0.0
MCU_VDD
2
YELLOW
D5
MCU POWER
R3
3
2
ADD WIRE
BETWEEN
JP1(1-2)
10
R2
GND
2
12MHz
Y2
1M
R10
VCC_SWITCH
DON’T
POPULATE
JP1
1
R21
VCC_USB
VCC_USB
VREG
2
C10 CLOSE TO J1
C6 CLOSE TO U1
C7 CLOSE TO USB SOCKET VCC
4.7uF
1
2
1
OSC1
OSC2
10K
R23
VCC_EXT
C10
1
MC68HC908JB16DW
/RST
PTA0
PTA1
PTA2
PTA3
PTE0
PTE2
PTA4
PTA5
PTA6
PTA7
PTD5
PTC1
/IRQ
VCC_USB
28
27
26
25
24
23
22
21
20
19
18
17
16
15
2
GND
1uF
C18
1
VCC_USB
DESCRIPTION: MC9RS08KA2 EVB+HB-LED DEMO (RS908KA2LED)
D6
1N5819HW
2
2
1
6
5
4
R20
MC34063AD
BDC_BKGD
BDM CONNECTOR
2
4
6
2
U8
SWCOL
DRCOL
SWEMIT SENSE
CAP
VCC
GND
COMPARE
VCCB
DIR
B
1
1
2
3
4
SN74LVC1T45DBV
VCCA
GND
A
J2
100
R18
1
2
BDM_IN_OUT 3
U2
GND
C11
2
600pF
1N5819HW
VCC_USB
2
1
D2
FREESCALE SEMICONDUCTORS HK LTD.
MCU_VDD
BDC_BKGD
BDC_RESET_VPP
C9
1
4.7uF
GND
2
1
VPP
2
1mH
2
1
L2
3
1
1
3
3
4
2N3904
Freescale Semiconductor
2
A
B
C
D
Appendix A.
Appendix A.
Schematic
Schematics
Q1
5
Troubleshooting
28
A
B
C
D
MCU_VDD
GND
BDC_RESET_VPP
1
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
5
FREESCALE SEMICONDUCTORS HK LTD.
DIM0
2
1
2
1
U3
4
750
2
1
DIM1
500
R26
ACMP+
8
7
6
5
ACMP+
ACMPPWM
DIM1
2
1
1
10k
100
R28
R27
MCU_VDD
3
2
1
D7
2
MBR130
1
1
Q4
L3
3
2
2
2
2
1
C21
0.1uF
330uH
NTR4502P
GND
3
2
2
1
DESCRIPTION: MC9RS08KA2 EVB+HB-LED DEMO (RS908KA2LED)
R25
10K
R24
3
4
PA0/K0/A+
PA1/K1/APA4/K4
PA5/K5
9RS08KA2DN
PA2/K2/RST
PA3/AO/BKGD
VDD
VSS
SW2
1
2
3
4
MCU_VDD
2
GND
JP3
2
0.1uF
C20
3
1
4
2
1
MCU_VDD
BDC_RESET_VPP
BDC_BKGD
5
100uF
2
1
C22
47uF
C23
GND
2
1
0.1uF
C24
2
WARM-WHITE
1.00
R29
1
2
1
REV: 0.0
D8
1
2
2
1
C25
1nF
1
DATE: 14/04/06
SHEET 02 of 02
33K
R30
1
A
B
C
D
Demo Setup
Troubleshooting
Appendix B.
Program Listing
;**************************************************************
;
; (c) Copyright Freescale Semiconductor, Inc. 2006
; ALL RIGHTS RESERVED
;
;**************************************************************
;**************************************************************
;* HB LED Coding for 9RS08KA2
;*
;* Author:
Vincent Ko
;* Date:
Apr 2006
;*
;* PTA0/KBI0/ACMP+
LED Driver
;* PTA1/KBI1/ACMPVoltage Reference
;* PTA2/KBI2/TCLK/RESETb/VPP
Dimming Button
;* PTA3/ACMPO/BKGD/MS
DIM0
;* PTA4/KBI4
PWM
;* PTA5/KBI5
DIM1
;*
;**************************************************************
; include derivative specific macros
XDEF
Entry
include "MC9RS08KA2.inc"
;=========================================================================
; ICS Definition
;=========================================================================
ICS_DIV_1
equ
$00
ICS_DIV_2
equ
$40
ICS_DIV_4
equ
$80
ICS_DIV_8
equ
$c0
;=========================================================================
; MTIM Definition
;=========================================================================
MTIM_DIV_1
equ
$00
MTIM_DIV_2
equ
$01
MTIM_DIV_4
equ
$02
MTIM_DIV_8
equ
$03
MTIM_DIV_16
equ
$04
MTIM_DIV_32
equ
$05
MTIM_DIV_64
equ
$06
MTIM_DIV_128 equ
$07
MTIM_DIV_256 equ
$08
MTIM_BUS_CLK
MTIM_XCLK
equ
equ
$00
$10
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
29
Demo Setup
MTIM_TCLK_FALLING
MTIM_TCLK_RISING
equ
equ
$20
$30
;=========================================================================
; ACMP Definition
;=========================================================================
ACMP_OUTPUT_FALLING
equ
$00
ACMP_OUTPUT_RAISING
equ
$01
ACMP_OUTPUT_BOTH
equ
$03
;=========================================================================
; RTI Definition
;=========================================================================
RTI_DISABLE
equ
$00
RTI_8MS
equ
$01
RTI_32MS
equ
$02
RTI_64MS
equ
$03
RTI_128MS
equ
$04
RTI_256MS
equ
$05
RTI_512MS
equ
$06
RTI_1024MS
equ
$07
;=========================================================================
; Application Definition
;=========================================================================
VREF
equ
PTAD_PTAD0
mVREF
equ
mPTAD_PTAD0
LED
equ
PTAD_PTAD1
mLED
equ
mPTAD_PTAD1
BUTTON
equ
PTAD_PTAD2
mBUTTON
equ
mPTAD_PTAD2
DIM0
equ
PTAD_PTAD3
mDIM0
equ
mPTAD_PTAD3
PWM
equ
PTAD_PTAD4
mPWM
equ
mPTAD_PTAD4
DIM1
equ
PTAD_PTAD5
mDIM1
equ
mPTAD_PTAD5
;=========================================================================
; Application Macro
;=========================================================================
Delay_1_cycle: macro
nop
;1 cycles
endm
Delay_2_cycles: macro
tsta
;2 cycles
endm
Delay_3_cycles: macro
brn
;3 cycles
endm
Delay_4_cycles: macro
brn
;3 cycles
nop
;1 cycles
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
30
Freescale Semiconductor
Troubleshooting
endm
Delay_5_cycles: macro
tstx
endm
Delay_10_cycles: macro
tstx
tstx
endm
Delay_15_cycles: macro
tstx
tstx
tstx
endm
;5 cycles
;5 cycles
;5 cycles
;5 cycles
;5 cycles
;5 cycles
org
TINY_RAMStart
; variable/data section
ONTime
ds.b 1
Pressed
ds.b 1
org
RAMStart
; variable/data section
org
ROMStart
; code section
main:
Entry:
;------------------------------------------------------; Config ICS
; Device is pre-trim to 20MHz ICLK frequency
; TRIM value are stored in $3FFA:$3FFB
;------------------------------------------------------mov
#HIGH_6_13(NV_ICSTRM), PAGESEL
;
mov
MAP_ADDR_6(NV_FTRIM), ICSSC
; $3FFB
;
mov
MAP_ADDR_6(NV_ICSTRM), ICSTRM ; $3FFA
mov
#ICS_DIV_1, ICSC2
; Use 10MHz
;------------------------------------------------------;Config System
;------------------------------------------------------mov
#HIGH_6_13(SOPT), PAGESEL
; Init Page register
mov
#(mSOPT_COPT|mSOPT_STOPE), MAP_ADDR_6(SOPT); BKGD disable,COP disabled
mov
#(mSPMSC1_LVDE|mSPMSC1_LVDRE), MAP_ADDR_6(SPMSC1); LVI enable
mov
#(RTI_128MS), MAP_ADDR_6(SRTISC); 128ms RTI
;------------------------------------------------------; Init RAM
;------------------------------------------------------clr
ONTime
clr
Pressed
;------------------------------------------------------; Config GPIO
; PWM - init H
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
31
Demo Setup
;------------------------------------------------------mov
#(mPWM), PTAD
; Initial port
mov
#(mDIM0|mPWM), PTADD
; DIM0 and PWM Output pins
;------------------------------------------------------;Config KBI
;------------------------------------------------------lda
#mBUTTON
sta
MAP_ADDR_6(PTAPE)
;Enable Pullup
sta
KBIPE
;KBI Enable
bset
KBISC_KBACK, KBISC
;------------------------------------------------------;Config ACMP
;------------------------------------------------------mov
#(mACMPSC_ACME), ACMPSC
; Enable ACMP
;------------------------------------------------------;Config MTIM
;
;Timer prescalar=1 -> Timer clk = 10MHz
;Bus = 10MHz
;Max OF period = 25.6us
;Timer resolution = 100ns
;------------------------------------------------------mov
#(MTIM_BUS_CLK|MTIM_DIV_1), MTIMCLK
mov
#255, MTIMMOD
;------------------------------------------------------;PWM Control Loop
;------------------------------------------------------PWMControlLoop:
bclr
PWM, PTAD
; Switch close
; --- Check Button --brset SRTISC_RTIF, MAP_ADDR_6(SRTISC), CheckPress;5
Delay_10_cycles
Delay_10_cycles
Delay_4_cycles
bra
CheckACMP
;3
CheckPress:
bset
SRTISC_RTIACK, MAP_ADDR_6(SRTISC);5
brset 0, Pressed, ToggleDim
;5
CheckButton:
brset KBISC_KBF, KBISC, ButtonPressed;5
Delay_5_cycles
Delay_4_cycles
bra
CheckACMP
;3
ButtonPressed:
bset
KBISC_KBACK, KBISC
;5
inc
Pressed
;4
bra
CheckACMP
;3
ToggleDim:
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
32
Freescale Semiconductor
Troubleshooting
bset
KBISC_KBACK, KBISC
lda
PTADD
;3
eor
#mDIM1
;2
sta
PTADD
;2
clr
Pressed
;2
Delay_3_cycles
;5
; --- Check ACMP --CheckACMP:
lda
ONTime
;3
brset ACMPSC_ACO, ACMPSC, LowerThanRef;5
HigherThanRef:
cbeqa #0, DummyCycles
;4
deca
;1
bra
PWMGen
;3
LowerThanRef:
cbeqa #255, DummyCycles
;4
inca
;1
bra
PWMGen
;3
DummyCycles:
nop
;1
bra
PWMGen
;2
PWMGen:
sta
ONTime
;2
bne
EnableOnTime
;3
Delay_10_cycles
Delay_3_cycles
bra
EnableOffTime
;3
EnableOnTime:
sta
MTIMMOD
;2 ON period
mov
#(mMTIMSC_TRST|mMTIMSC_TOIE), MTIMSC;4 Reset and Start Timer
lda
#255
;2
sub
ONTime
;3
wait
;2+
beq
ZeroOffTime
;3 Duty cycle is off
EnableOffTime:
bset
PWM, PTAD
;5 Switch open
sta
MTIMMOD
;2 OFF period
mov
#(mMTIMSC_TRST|mMTIMSC_TOIE), MTIMSC;4 Reset and Start Timer
wait
;2+
jmp
PWMControlLoop
;3
ZeroOffTime:
Delay_10_cycles
Delay_3_cycles
jmp
PWMControlLoop
;3
;%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
; Reset Vector
;%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
org
$3ffc
Security:
dc.b
$FF
jmp
main
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
33
Demo Setup
High Brightness LED Controller using the MC9RS08KA2, Rev. 0
34
Freescale Semiconductor
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